The catalytic converter has significantly reduced pollution from vehicles since it was introduced more than thirty years ago, but what is really impressive is that it costs nothing to run and does not wear out. Engine exhaust gases are converted into less harmful gases inside the converter as they pass over a honeycomb coated with precious metals. The two main types of catalytic converter are the three-way and the two-way. The three-way converter is mostly used with gasoline engines, while diesel engines use the simpler two-way converter. Both types normally last for the lifetime of the vehicle, but they can be ruined by using leaded fuel or fuel additives. Since they are very expensive to replace, this should be avoid as much as possible.
Many countries now have environmental laws that require the use catalytic converters to reduce smog and other forms of vehicular pollution. They are found on most new vehicles that use unleaded or diesel fuel but not on older vehicles that need leaded fuel. There have even been calls for catalytic converters to be installed on lawnmowers because they produce a significant amount of pollution. Converters can change almost all the carbon monoxide and hydrocarbons present in the exhaust gases into less harmful carbon dioxide and water, and the three-way converters can even convert nitrogen oxides into other harmless gases.
Catalytic converters are located on the exhaust pipe underneath the car, between the engine and the muffler. They either resemble a smaller muffler or an expanded section of pipe, and their case is usually made of stainless steel.
Exhaust gases from the engine enter at one end and pass straight through the honeycomb structure before exiting from the other end. Installing a new converter is a simple task that takes about the same time and effort to change a muffler. The old converter can even be sold to scrap metal dealers who pay good money for them, because they contain precious metals.
Catalysts are substances that start a chemical reaction or make one go faster. They are not consumed in the reaction because they are used and replaced at the same rate, a process called the catalytic cycle. The precious metals used for the oxidation reaction are platinum and palladium, while platinum and rhodium are also used in three-way converters for the reduction reaction. These very rare metals are the main reason that converters are so expensive. Other cheaper metals can be used but they don't work as well, and it is illegal in some countries to use nickel and copper.
The two-way catalytic converter used with diesel engines performs two oxidation reactions. Carbon monoxide is oxidized into carbon dioxide, and hydrocarbons from unburnt fuel are oxidized into carbon dioxide and water. The converter cannot remove the large amount of carbon soot in diesel exhaust so a filter is installed before it. The filter automatically removes the soot by burning it off with a small amount of diesel fuel at regular intervals.
The three-way catalytic converter used with gasoline engines performs the same two oxidation reactions but also performs a reduction reaction. It has a second stage coated with different metals that convert nitrogen oxides into harmless nitrogen and oxygen. While the efficiency of this reaction is slightly less than that for the oxidation reactions, almost all of the nitrogen oxides are removed from the exhaust.
Catalytic poisoning refers to the coating of the precious metals inside the converter with contaminates. This reduces the conversion efficiency because there is less contact area between the metals and the exhaust gases. The lead content in leaded petrol can quickly ruin a converter so it is important to always use unleaded fuel. Before adding fuel additives to the tank, check the label to make sure they are safe for use with catalytic converters.